1. Field of the Invention
This invention pertains generally to wireless communication, and more particularly to reducing RF fading in wireless communication systems.
2. Description of Related Art
Wireless communications have proliferated in recent years because of their mobility and convenience. The basic feature of wireless communication is transmitting and receiving RF signals through the air, without wires, often between a base station and a mobile station. One particular type of wireless communication system is the wireless local area network (WLAN). WLANs are built according to a number of standards, particularly several 802.11x IEEE standards. Information is typically sent as packets, containing identifying information, the actual information, and error information. The complete message may be contained in a number of different packets.
Whatever type of wireless system is used, a common requirement or goal is high performance. These systems all face performance problems associated with RF propagation. Signal variation due to RF propagation problems will negatively affect system performance.
RF propagation (e.g. RF propagation of signals transmitted from 802.11 WLANs) encounters spatial as well as temporal fading. The causes of fading include constructive and destructive interference of RF due to multipath propagation, as well as the motion of objects in the environment. Such fading can cause the power of an RF signal to vary by several dB over distances of an inch or more, in addition to variation of signal power over time at any single location.
RF fading is a problem for wireless systems based on technologies such as MIMO (Multiple Input, Multiple Output). In such MIMO systems, spatial multiplexing is used to increase the capacity of a single frequency channel. Data is transmitted from two or more antennas simultaneously, and the data on each antenna is different. For example, by using three transmit (Tx) antennas and three receive (Rx) antennas, the spectral efficiency (i.e. capacity) of an 802.11 channel may be increased 3×. However, performance of MIMO systems depends on the nature of the signal received at each of the three Rx antennas from each of the three Tx antennas. Ideally, the paths from each Tx antenna to each Rx antenna are uncorrelated while having sufficient signal to noise ratio (SNR) to allow reliable demultiplexing at the receiver. However it is possible that the signal at one or more of the three antennas at the receiver will have a low SNR (Signal to Noise Ratio) and hence will be unable to support a desired data rate for the MIMO system (e.g. 72 Mbps) even though a few (e.g. three) inches away a signal with adequately high SNR may exist.
RF fading is also a problem for WLANs using coherent combination at the receiver, such as those employing Maximum Ratio Combining (MRC); these can be MIMO or SIMO (Single Input, Multiple Output) systems. In cases where MRC-type processing is used with MIMO, the data is transmitted from the transmitter from two or more antennas simultaneously; however, the data on the Tx antennas is the same sequence, perhaps only offset by a fixed time delay. Here, additional (>1) receivers are used to increase the SNR at the receiver while attempting to avoid fading by spatially separating the receiving antennas.
It is sometimes possible to improve performance of the receiver during fading by simply moving the receiver in its local vicinity. However this is not practical in cases where the receiver is part of a large immovable object, and is also not user-friendly since it is often unclear to even a mobile user exactly how or in which direction the receiver should be moved in order to improve performance.
In order to help mitigate fading for MIMO as well as MRC-type systems, additional Rx paths are usually added. Each additional Rx chain includes not only a dedicated antenna but also dedicated Low Noise Amplifiers, PHY (RF and digital) chips, and other components. The signals from each additional antenna (processing path) are processed at the receiver. By adding these additional processing chains in parallel to those existing previously in the system, the receiver can improve SNR of the received signal while also sampling RF from spatially separated locations, thus decreasing the possibility of fading affecting all the antennas simultaneously. Hence additional antennas can provide spatial, polarization, pattern, and other types of diversity that improve performance in MIMO and MRC-type wireless systems. However, the problem with this approach is that adding additional parallel processing chains is computationally complex, and adds many more components, and is hence more expensive and less compact to implement.
Accordingly it is desirable to provide improved method and apparatus to reduce RF fade in wireless communication systems.